It's frustrating. He has a difficult time doing even a goblet squat without curving his spine over. I don't want to start him on the pulls and swings (which he does ok sometimes) without first making him aware of this.

I don't even want to talk about the upper body strength of your average American 13 year old. Let's just say that 'girl' push ups are challenging.

Dude, we have too much in common. I can't even get my 13 y-o daughter to lift her ass off the couch, let alone deadlift. I have taken her to the gym a couple of times and showed her the DL. Girls seem to get the neutral spine easier than boys, who want to use the lower back to lift everything.

I might use the Dan John method of teaching the RDL. Hold a pole behind him and have him stick his butt out to touch it. Keep moving it back and have him stick his butt out farther. Eventually, he'll get it.

The 12 year old has no interest in what I do, but then he begs to use the bowflex (my wife's) so he can get buff. Drives me nuts, so I set a rule down that he has to be able to do certain thigns to standard before I even let him touch a bowflex i.e. pushups, pullups squats...etc

The 8 year old is very interested in all of it and as long as I remind him he can squat and do swings without rounding his spine. When he does squats I always remind him to sit back like sitting into a chair. With swings I tell him to keep his shoulders back (as opposed to huncing over) and to sit back.

__________________
"And for crying out loud. Don't go into the pain cave. I can't stress this enough. Your Totem Animal won't be in there to help you. You'll be on your own. The Pain Cave is for cowards.
Pain is your companion, don't go hide from it."
-Kelly Starrett

the ghd extensions are good. even starting with a superman to get that feeling of contracting the glutes and erectors in a virtually no-movement, easy position. then standing, trying to set the back the same way and incrementally leaning over farther and farther. have him stick his hands in the creases of his hips (pinky sides, like he's judo-chopping himself) to provide input on where he should be folding.

finally, reinforcement. when he does it right, give him a cookie. when he does it wrong, hit him with the kettlebell.

Demonstrate the technique and let him copy it, do it often, very often, be the model. With respect to the swing teach it like rocking a swing, very little amplitude first, think centimeters, the movement is pretty much self-correcting.

I've sometimes had people start with just a waiter's stretch to get that glute/ham awareness. Sometimes if they're really struggling I'll have them standing with their back to a wall or door, so that they can push their hips back to rest against it while I mess with their body position if necessary. Once they can do that and feel the stretching and activation, they'll progress on to the supermans, BE, etc., that everyone has already mentioned.